Deemed to Satisfy Provisions for Corrosion Protection of Structural Steel (NCC 2022)

NCC 2022

There have been significant changes to the corrosion protection provisions in NCC 2022. These changes are now effective across all of Australia. Part 6.3.9 of the ABCB Housing Provisions, Corrosion protection, covers the corrosion protection requirements for all structural steel not built into a masonry wall. There are now three tables within the clause which are required to be followed to achieve a Deemed-to-Satisfy (DtS) solution, although only Table 6.3.9a is required for hot dip galvanizing alone.

Hot dip galvanizing must be in accordance with AS/NZS 2312.2, the design and durability Australian Standard for hot dip galvanized coatings manufactured in accordance with AS/NZS 4680. Duplex coatings must be in accordance with AS/NZS 2312.2 and these are detailed in Table 6.3.9c. Paint systems must be in accordance with AS 2312.1 and these are detailed in Table 6.3.9b.

With these three additional compliance tables, corrosion protection specifications direct from the AS 2312 series are implemented for the NCC and offer a wider range of DtS solutions than previously available.

The table below shows the relationship between the minimum requirements for hot dip galvanizing to AS/NZS 4680 and the requirements in Table 6.3.9a.

For sections highlighted in green, all steel conforming to AS/NZS 4680 will meet the DtS requirements of the NCC. Where the sections are highlighted in yellow, a duplex coating will be required for a DtS solution. The orange sections indicate only some hot dip galvanized products are suitable and more information is provided in the notes and the sections following the table.

It is the responsibility of the designer to determine the environment and specify the steel thickness required for structural adequacy. We recommend reading about durability of hot dip galvanizing here or in the GAA's Advisory Note AN 49 (requires a one-time registration to access).

The galvanized coating thickness formed on steel is directly related to the steel thickness. For some steels additional processing may be required to reach the desired specification. The GAA and your galvanizer can provide additional advice.

HDG900 will not be available for all steel thicknesses and steel grades and should only be ordered for special applications. As shown in the key to the table, HDG900 availability normally occurs when the steel thickness is more than 10 mm, and the silicon content of the steel is more than 0.01%. Check with the galvanizer before specifying by providing them the section information or steel drawing.

To check the coating thickness supplied, a simple non-destructive test is possible. If a test report is required, this must be included on the order and will usually incur a cost, otherwise a certificate of conformance can be supplied by all members of the Galvanizers Association of Australia on request.

Duplex Systems to AS/NZS 2312.2

Duplex systems are simply standard hot dip galvanized steel, with a specified paint coating applied. The galvanized steel will always need roughening and cleaning to provide adequate adhesion of the paint to the galvanized surface. The three systems in the Housing Provisions are shown below.

2D Degrease, wash, and dry, then sweep blast and paint with 2-pack inhibitive epoxy primer to 75 µm DFT plus polyurethane or 2-pack acrylic gloss to 100 µm DFT.

4D Degrease, wash, and dry, then sweep blast and paint with 2-pack high build epoxy to 250 µm DFT plus polyurethane or 2-pack acrylic gloss to 100 µm DFT.

5D Degrease, wash, and dry, then sweep blast and paint with 2-pack epoxy primer (inhibitive) to 75 µm DFT plus 2-pack high-build epoxy to 225 µm DFT plus polyurethane or 2-pack acrylic gloss to 100 µm DFT.

Notes:

Surface preparation is critical to long life performance of painted surfaces and sweep blasting must be carried out by an expert.
DFT = dry film thickness. This means the thickness of each paint layer measured after it has dried.
Do not use the sweep blasting procedure on continuously galvanized hollow sections as it will excessively damage the coating. Sweep blasting preparation is only suitable for steel hot dip galvanized to AS/NZS 4680.
All major paint suppliers can provide paints to these specifications. Paint manufacturer instructions must be followed to ensure adequate corrosion protection.
Options for duplex coatings exist in Table 6.3.9a. For example, HDG150 & 4D or HDG300 & 2D are alternative deemed-to-satisfy solutions for a High (C4) environment although the thinnest commercially available batch HDG coating is HDG320.
The GAA has a Guide to Painting and Duplex Coatings - Paint Products Guide available for free download from our Technical Publications page (requires a one-time registration).

Environment and Description

There are four external environments described in the new Housing Provisions for structural steel. These are low, medium, high and very high and are described as C2, C3, C4, and C5 in AS/NZS 2312.2. If the steel is internal and not otherwise exposed to the external environment, corrosion protection may not be required. Posts, stumps, piers, and sub-floors are not internal environments, nor are the steel elements built into masonry walls or other structural steel elements in contact with an external wall. Professional advice is usually required to define an internal environment.

A low environment is typically remote inland areas (more than 10km from breaking surf) or more than 1km from a sheltered bay, including dry rural areas remote from the coast or sources of pollution. Many areas of Australia beyond at least 50 km from the sea are in this category, including most cities and towns such as Canberra, Ballarat, Toowoomba, Alice Springs and some suburbs of cities on sheltered bays such as Melbourne, Hobart, Brisbane and Adelaide that are more than 1 km from the sea. However, each of these have many exceptions which are in more corrosive categories.

A medium environment is typically more than 1km from breaking surf or heavy industrial areas or more than 50m from salt water not subject to breaking surf, typically around sheltered bays, such as Port Phillip Bay. This extends from about 50 m from the shoreline to about 1 km inland but seasonally or in semi-sheltered bays extends 3 to 6 km inland. Along ocean front areas with breaking surf and significant salt spray, it extends from 1 km inland to about 10 to 50 km depending on wind direction and topography. Much of the metropolitan areas of Wollongong, Sydney, Newcastle, Perth and the Gold Coast are in this category. This can extend to 30 to 70 km inland in South Australia while on some evidence, other southern Australian coastal zones are in this, or a more severe category. This also includes urban and industrial areas with low pollution and for several kilometres around large industries such as steel works and smelters.

A high environment is typically more than 200m from breaking surf or 50m from sheltered bays. In areas of rough seas and surf it extends from several hundred metres to about 1 km inland. As with other categories the extent depends on wind, wave action and topography. The category will also be found inside industrial plants and can influence 1.5 km down wind of the plant.

A very high environment is typically within 200m of breaking surf or a heavy industrial area. It is also found in aggressive industrial areas with a pH of less than 5.

Breaking surf is defined in the NCC as any area of salt water where waves break on an average of 4 days per week but does not include white caps or choppy water. Breaking surf normally occurs in open seas and would usually preclude sheltered locations in the vicinity of Port Philip Bay, Sydney Harbour and near coastal rivers such as Derwent, Swan, and Brisbane Rivers.

Heavy industrial areas are typically found in the industrial environments around major industrial complexes. Corrosion of steel from industrial effects is no longer an important factor in housing as heavy industrial areas are relatively few in Australia and are known from surveys to be restricted to the areas described in AS/NZS 2312.2.

More Information

More information on best practice design to achieve the expected durability of hot dip galvanized coatings described in the Housing Provisions Standard is available in our Advisory Note AN 49. This is essential reading for professionals looking to develop a Performance Solution. We also have published a Guide to Painting and Duplex Coatings - Paint Products Guide for those looking to understand more about painting hot dip galvanized steel.

All documents are available for free download from our Technical Publications page (requires a one-time registration).

The Queensland Building and Construction Commission have published a Fact Sheet providing guidance on this subject.

NSW Fair Trading have published guidance on this subject at Certifier requirements for corrosion protection of steel posts.

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